Cheese varieties such as Swiss cheese typically contain voids or round holes in the cheese commonly referred to as eyes. The voids or eyes are the result of carbon dioxide produced by eye-forming starter bacteria (e.g., Proprionibacterium fruedenreichii subsp. shermanii). The manufacture of Swiss cheese with consistent eye formation is both science and art. There are many factors that affect the number, size, appearance and distribution of eyes. Bacteria viability and concentration, curd elasticity, imperfections in the cheese, lactate concentration, temperature, and duration in a warm room are just a few.
Cheese makers using eye-forming bacteria must wait several weeks or more for the eyes to form. The cheese can not contain a high level of salt because it would prevent growth and carbon dioxide production by the eye-forming bacteria. Further, the cheese can not be subjected to temperatures above about 130° F. because the bacteria responsible for producing carbon dioxide gas would be destroyed.
In addition, natural cheeses that are made without eye-forming bacteria typically do not form eyes. Further, the processing conditions typically associated with process cheese (e.g. heat treatments and emulsifier levels) typically prevent eyes from forming even in the presence of cultures.